The psbA genes of plant chloroplasts and cyanobacteria encode a polypeptide which is an integral protein of the thylakoid membrane. This protein, in complex with a quinone molecule, serves as the second stable electron acceptor for photo-system II, termed Q-B. The Q-B protein is also the target for the herbicides atrazine and diuron. Several plant and cyanobacterial species have been shown to exhibit herbicide resistance due to a mutation in the psbA gene which causes a single amino acid alteration in the protein. The psbA gene is a unique gene in the chloroplast genome of higher plants. The transformable cyanobacterium Anacystis nidulans R2 has three non-identical psbA genes in its genome, and an allele of one of these has been shown to confer herbicide resistance. This project will investigate the expression of the three genes by using Northern analysis with copy-specific probes to detect transcripts and gene fusions to the lacZ gene to study translation in vivo. The properties of the protein product of each gene will be studied using strains in which two of the three genes have been inctivated, allowing the measurement of photosynthetic parameters in the presence of a single Q-B species. Specific regions of the polypeptides which interact with other components of the photosynthetic apparatus and with inhibitors will be examined by in vitro mutagenesis, followed by recombination of the mutated genes into the A. nidulans chromosome. The dominance properties of the herbicide resistance allele will be investigated by studying the sensitivity of strains which are diploid and heterozygous for herbicide-resistance at the psbAI locus and have the other psbA genes inactivated. The results of this study will benefit two major areas of scientific investigation: the regulation of multigene families and the structure and function of the photosynthetic apparatus.